The present invention relates to x-ray computed tomography (CT) and specifically to a CT system and method providing high-speed data acquisition.
Conventional x-ray computed tomography may employ an x-ray source collimated to produce a narrow fan-shaped beam directed along the transverse plane through a patient to be received by linear multi-element detector array.
The x-ray source and detector array may be mounted on a gantry to be rotated about a patient to obtain “projections” measuring x-ray attenuation at the different gantry angles along a slice plane though the patient. A “projection set” of such projections, for example, of projections obtained over a range of gantry rotation of 180 degrees plus the angle subtended by the fan beam, may be “reconstructed” into a tomographic image. The tomographic image shows a cross section of the patient along the slice plane.
The collection of a projection set can be viewed as an acquisition of lines of data in “k-space”. Each projection (i.e. the x-ray attenuation values measured by each element of the multi-element detector at one gantry angle) provides one line of data in a k-space plane at the same gantry angle and in the slice plane. A projection set provides a series of lines of k-space data extending like spokes of a wheel from the center of k-space. K-space is a frequency domain version of the tomographic image, with data near the center of k-space representing low-frequency image data and data at the edges of k-space providing high-frequency image data. A two-dimensional Fourier transform of k-space data collected by the projections provides the tomographic image.
Reconstructions using too few projections or a limited number of gantry angles may produce images with “artifacts”, typically streaks, that mar the tomographic image.
In a normal CT acquisition, multiple projection sets are obtained along different, sequential slice planes by movement of the patient within the gantry. The slices may be assembled to provide data for an arbitrary volume of the patient which may then be reconstructed into cross-sectional images along arbitrary planes.
The time required to collect projection data over a volume using conventional fan-beam tomography can be substantial and may preclude the use of tomography in situations where large volumes are to be monitored in real-time (for example, in contrast studies) or where there is unavoidable patient or organ motion during the acquisition time.
One method of increasing CT acquisition speed is by using a spiral or helical scan in which the gantry containing the x-ray source and detector is rotated continuously as the patient is moved. This process eliminates the need for stopping and starting the gantry and table.
Wider fan beams may also be used with rectangular detector arrays having, for example, 500 or 1,000 detector elements in the scan plane and sixteen detector elements perpendicular to the scan plane. This arrangement allows multiple slices to be collected at one time or allows for a steeper helical pitch.
While these systems provide for more rapid data acquisition, they are fundamentally limited by the rotational speed of the gantry. This latter limitation has been addressed by “electron beam” systems in which a steerable electron beam scans a hemi-cylindrical anode array providing what is in effect an x-ray source rotating about the patient within a plane but without mechanical movement.